Process for removing zinc from zinc oxide containing material



Aprll 1, 1958 H. CLASEN 2,829,044

v PROCESS FOR REMOVING ZINC FROM ZINC OXIDE CONTAINING MATERIAL Filed May 1o, 195e PROCESS FOR REMOVTNG ZINC FROM ZINC XIDE CNTANING MATERIAL Hermann Clasen, Frankfurt, Germany, assigner to Metallgesellschaft Aktiengesellschaft, Frankfurt, Germany, a corporation Application May 10, 1955, Serial No. 584,124

Claims priority, application Germany May 28, 1955 33 Claims. (Cl. 75-86) This invention relates to a process for the removal of zinc from oxide materials, particularly from such materials which contain iron and zinc, if desired in conjunction with the recovery of metallic z inc.

Such materials occur in large quantities. They include, e. g., franklinite containing, on an average, 21% zinc oxide, 59% Fe2O3, 11% FeO and 8% ,Mn2O3. The process according to the invention is, also applicable to materials which are not available initially in the form of oxides but can be transformed into the same.v This applies particularly to sulfide or carbonate ores. For instance, there are ce-rtain pyrite deposits which contain sphalerite, or yiron-containing smithsonite containing up to 50% FeCO3. These ores can be transformed in tc oxides by the known roasting calcining and sintering processes.

. Meggen pyrites are-particularly important for German conditions. They contain about 32% Fe, 8% Zn, 40% S, SiOz. The' iron is in the form of pyrites, the zinc in the form of blende. Both materials are intimately intergrown. In the sulfuric acid plants the Meggen pyrites are roasted down to a residual sulfur content of a` few percent so that the calcine is mainly in the form of oxides.

The usual process for the thermal recovery of metallic zinc from oxide materials consists of reducing the zinc oxide with a surplus of carbon in the horizontal or vertical retort. According to the state of the art a recovery of zinc by that process is economical at the present price of zinc only with materials which contain more than 40% zinc. In oxide materialshaving a low zinc content, such as calcine from Meggen pyrites, vwhich con tains about 8-10% zinc and about four times that amount of iron, no metallic zinc is obtained by that process and zinc is removed only to a very incomplete extent because the material sinters together in a considerable degree at the temperatures applied. Materials having a high iron content are unsuitable for being smelted` in the horizontal retort also because they sinter heavily and the retort suffers great wear.

Zinc oxide and zinc ferrite have also been mixed with metallic iron and metallic zinc has been driven off from such mixtures in a nitrogen stream at 1000 deg. C. (cf. Meltall und Erz, 26 (1929), pp. 356-357) or has been recovered in a vacuum (German Patent No; 504,227). In these processes the reduction takes place in accordance with the equation 4ZnO+3Fe=Fe3O4+4Zn in the case of zinc oxide, and according to the equation ZnO.Fe2O3{-Fe=Fe3O4-}Zn in the case of zinc ferrite.

When the zinc has been transformed into an oxide by roasting, the resulting temperature rise to about 900 deg. C. cannot be utilized for the thermal removal of zinc but an intermediate cooling and reheating must be applied, because the iron, which is preferably in a finelykdivided United States ,Patent OV y BatentedApx-, 1, 1958.

state, cannot be admixed homogeneously at 900 deg. C.

It has now been found that it is not necessary to use metallic iron for reducing the zinc but that the reduction of zinc can be effected also with iron oxide whose mole ratio Fe:O exceeds 3:4 and amounts, e. g., to about 1:1. Such a process is of advantage for the removal of zinc or the recovery of metallic zinc from materials which contain zinc oxide and iron oxide. In such materials the iron oxide can be reducedin a preliminary reduction stage to such an extent that a sufficient amount of free lFe() is contained in the oxide mixture. The preliminary reduction need not be vcarried to such an extent that the mole ratio of metallic iron and iron-combined oxygenis exactly 1:1 but that ratio may be higher or lower than said value, which corresponds to the compound FeO (wstite)A The degree to which the preliminary reductionis carried willV depend in the first place on the fact whether the material to be dezincked contains in additionto zinc oxide a sufiicient amount of iron oxide. To reduce the zinc according to the equation three parts of FeO are required per part of ZnO in order to drive off'the zinc in zinc oxide reducing stage. This means that the material must contain at least the amount of FeO required according to that equation. In the case of arsufiiciently high content 4of iron oxide, therefore, the mole ratio Ee:O may beso-much lower thanlzvly that` this minimum'content of free FeO isY available. vSince the compound F6304 may be considered also as FeO.Fe2O3, care must be taken that only the FeO which is present in addition to the compound Fe304, i. e. is

Y in addition to the molera'tio FeO:Fe2O3=1:1, can become effective. must be reduced untilthe FezO mole ratio exceeds 3:4. The mole ratio of Fe and iron-combined' oxygen may exceed the value 1:1. In that case the capacity to absorb oxygenincreases. rA ratio higher than 1:1 will be par-` ticularly desirable when the iron content related to the zinc oxide content would not be sufficient for reducing allzinc oxide if the preliminary reduction vwas carried only to an Fe:0 mole ratio of 1:1. According to the known iron-oxygenphase diagram, metallic iron (more exactly oxoferrite or oxoaustenite) will be present beside FeO (moreprecisely wstite) if the mole ratio is higher than 1. Y

Therefis also an upper limit' because the volatilization of zinc to form a partly oxidized powder increases owing to the higher reduction temperature and longer reduction time involved inan increased reduction to metallic iron. To enable the recovery of the largest amount of compact, oxide-free zinc during the subsequent dezmcking stage, the oxidized' zinc which has been prematurely volatilized during the preliminary reduction stage ought to be admixed homogeneously before the dezincking stage. This involves difficulties and expenses, particularly if the'charge whichy has been preheated during the preliminary'reduction stage is to be fed to the dezincking stage without cooling. The partly oxidized zinc which has been prematurely volatilized during the pref liminary reduction stage cannot be admixed to' a new charge. to be subjected to preliminary reduction in order to utilize inv the dezincking stage the preheating 'effected during the preliminary reduction stage because'this would ly high rate, it may be suitable to reduce first only part ofthe zinc oxidewithfthe FeO which has been obtained during'the preliminary reduction stage without appreci- This 'means that the starting'V mixture Fe304 to another preliminary reduction, and finally to complete the reduction of the zinc oxide. It is also possible to add iron oxide in a suitable form (preferably with a mole ratio Fe:O 3:4) and/or metallic iron in order to have the reducing agent for the zinc oxide available in a sufliciently large amount.

In the accompanying drawings,

Fig. 1 is a phase diagram plotting the content of metallic iron against the content of divalent iron in the material before the zinc oxide reducing stage.

Fig. 2 is an embodiment of an apparatus suitable for reducing zinc oxide according to the teaching of the invention. v n t The contents of Fe203, FeO and Fe, and with it the mole ratio Fe:O are usually determined byy a determination of `the total iron content, of the content of trivalent iron (dissolving in the absence of air by means of sulfuric acid and titrating according to Reinhard-Zimmermann) and the metallic iron (e. g., according to the HgClg method).

When

Moles total zinc 1 Moles total iron=3 and the reactions Y the degrees of dezincking apparent from Fig. 1 are to be expected for the ascertained contents of divalent iron related to the `total iron content, and of metallic iron, related to the total iron content.

In Fig. 1 theabscissa represents the content of divalent iron related to the total iron content and the ordinate is the content of metallic iron related to the total iron content before .the beginning of the zinc oxide reducing stage. When no metallic iron but only divalent iron is available as reducing agent, BB1/3% divalent iron will not yet be sufficient for dezincking because there is no free FeO present which causes the reduction of zinc oxide since the trivalent iron also present can combine with the entire amount of divalent iron according to the equation On the otherV hand, if there is no divalent iron present but only metallic iron and trivalentiron, a removal of zinc can be expected only with a metallic iron content of at least 11i/6%u because metallic iron up to 11%% will be consumed by Fe2O3 according to the equation If the material contains Fenrand metallic Fe in addition to FeIn before the zinc removal stage and if the contents of Fe1I and metallic Fe are below the straight line which leads from 33%% FeII to 11%% metallic Fe, no reduction of zinc oxide will be obtained.

A 100% removal of zinc can be obtained in the absence of metallic iron if the content of divalent iron amounts at least to i les total zinc content 1 2..r

. moles total iron content-F3 100% The corresponding `value for the content of metallic iron required in the absence of FeII is 1A of the above value. It the material to be dezincked contains metallic iron, divalent and trivalent iron, a 100% remQVal of zinc can be obtained with all compositions which lie above the straight line leading from the point to the point moles total zinc content 1 C. moles total iron contlenbi--)fmo moles tot-al zinc content l moles total ion content 3 100% The area between these two straight lines is the area in which only part of the zinc is removed. In that area the curves denoting equal degrees of zinc removal are straight lines which are parallel to the two straight lines defined. The distance of said straight lines from the straight line denoting 0% removal of zinc is proportional to the degree of zinc removal.

moles zine eontent+ 1 .00%

is either necessary to carry the reduction of the iron oxide to a higher degree so that part of the iron is reduced to metallic iron, or iron must be added which is used in the form of metallic iron or ironIl for the reduction of the zinc oxide or the reduction of the zinc oxide must be performed in several stages.

For instance, if the oxide material to be dezincked has a zinc content of 10.2% by weight and an iron content of 42.3 by weight, the mole ratio When the preliminary reduction stage is controlled so as to avoid a volatilization of zinc that ratio can apply also to the pre-reduced material. That ratio is multiplied with 2 and added to 0.333, giving 74.5%. This means that for a removal of zinc the preliminary reduction of the starting material must be carried at least t0 such a degree that about 75% of the iron is present in the form of ironII or that the share of divalent iron in the total iron content together with three times the share of metallic iron in the total iron content must be at least 75 a1 Zinc iron: =0.206

Fell

From Fig. l itis apparent that threee moles` of FeO can reduce as much zinc oxide as one mole of metallic iron. Since, moreover, the rate at which zinc is being removed when three mole FeO act on 1 mole zinc oxide at temperatures below about 1000 deg. C. is not lower but even somewhat higher than when 1 mole of metallic iron (of the same degree of distribution) is effective, the area in which zinc is being removed mainly by FeO is at least the field which is enclosed by the thick line in Fig. l. For this field Fell Femet.

Fe Fe The preliminary reduction of the iron oxide present in a mixture with material which contains zinc oxide can be effected in various ways. Carbon, e. g., in the form of anthracite, may be used for this purpose. The carbon is preferably used in such an amount that it is consumed after the preliminary reduction so that the subsequent reduction of the zinc oxide and the removal of the zinc can be effected by-heating in a vacuum without disturbance-vof' thevacuum'by CO or CQzformed by the residual carbon andthe-oxygen of the oxide.

It is also possible to perform the reduction with the aid'of gases to obtain a carbon-free product by the preliminary reduction so that a detrimental action yon the vacuum during the reduction of the zinc oxide is avoided.

Pure hydrogen or p-ure CO or hydroreduction is preferably performed at a low temperature. Y, When performed `with carbon, the preliminary reduction may be eiected at950 deg. C. without Volatilization of appreciable amounts of zinc during the necessary heating time. Whereas iron oxide is reduced also at temperatures below 900 deg. C. the time required in that case tends toy involvea volatilization of more zinc than a preliminary reduction performed in a shorter time deg. C.

A great advantage of the process according to the in-y vention resides in that the preliminary reduction need not be continued until a considerable amount of metallic iron is obtained because the times and/or temperature conditions which would be required therefore will cause in most cases a considerable volatilization of zinc, the zinc being obtained as a low-grade dust which contains larger or smaller amounts of zinc oxide and which can be processed to zinc only with diiculty. Moreover, that volatilization of zinc during the preliminary reduction stage reduces the zinc content of the charge so that the processing by the actual reduction of zinc oxide is less profitable. n

If the preliminary reduction is performed with the aid of reducing gasesyit is recommendable to elect the reduction at about 800 deg. C. At temperatures over about 9.00 deg. C. the volatilization of zinc in the gasr stream is so strong that the process can no longer be divided into a preliminary reduction stage and a zinc oxide reducing stage. In the case of a reduction with gases the temperature applied during the preliminary re duction stage must adapted to the velocity of the reducing gases, or vice Versa, because the flowing gas will at a higher temperature carry off in a higher degree anyv zinc formed by the reduction.

The pre-volatilized zinc obtained with a high oxide content cannot readily'be returned as oxide-containing zinc material into the second, actual zinc oxide reducing stage. For this reason the volatilization of zinc in the preliminary reduction stage should be avoided as far as possible. To enable theprocessing of that oxide-containingzinc in the zinc oxide reducing stage at least part of ythe pre-reduced material would have to be cooled and the' oxide-containing zinc would have to be admixed thereto in the cold. This would require a reheating of that mixture.

Y f Where CO-containing industrial gases are used the preliminary reduction can be carried in all cases to a` point where Fez0=1:l, without an appreciable removal of zinc. The removal of zinc can be kept below 1% ot the zinc content. When carried farther the preliminary' reduction will frequently involve a greater removal of zinc unless hydrogen gas at 800 deg. C. and ata low velocity of ilow is used as a preliminary reducing gas. By such a preliminary reduction stage a high content of iron, i. e. of about 75% metallic iron in -addition to the FeO, can be obtained, Without causing a volatilization of zinc amounting to more than 1% of! the zinc content in the preliminary reduction stage. Where gases having a high hydrogen content are employed in a shaft furnace for the preliminaryIv reduction stage about metallic iron can be obtained with a volatilization of 1% vof the zinc. For this reason hydrogen would be particularly suitable for this purpose but has the disadvantage that its price is high and that the rate of zinc removal is low owing to the low ilow lrate required and the limitation of temperature'.' f

The concentration of the reducing agent increases with the content vof metallic iron (l mole of iron being equivalent to 3 moles of FeO). This causes an increase in the velocity of the subsequent reduction of zinc oxide and in the iron content of the dezincked material so that the dezincking is accelerated and a higher price can be obtained for the dezincked material. These advantages,l however, arev opposed by the disadvantage that fairly high costs are involved in obtaining high contents of metallic iron. For this reason it is most economical in most cases to use divalent iron in a larger amount than metallic iron. Since metallic iron is not required in the process according to the invention, very inexpensive gases having a high CO2 content (e. g. a carbon monoxide gas containing more than 35% of CO2 by volume at 800V deg. (l), which cannot give any metallic'iron during the preliminary reduction, can tbe used in the preliminary reduction stage as a reducing agent instead of the ex-v pensive gases having a large hydrogen content.

The preliminary reduction can be accelerated by the presence of, e. g., steam and/or additions of about 1% ot' soda, ferrie chloride, chlorine whereby thevolatilization of zinc can be reduced in most cases.

When the gas flows at a low rate the volatilization of zinc during the preliminary reduction may be retarded by' steam, which may also `be added to the reducing gas, also because it will oxidize the zinc Vapor at a temperature which is not excessively high, e. g. in the colder upper part of the charge in the shaft furnace, the zinc oxide being then filtered by the charge from the gas stream. Carbon dioxide has a similar reoxidizing eect on zinc vapor.y lThus the Volatilization of zinc can be kept down when a high concentration of H2O and/or CO2 is maintained during the preliminaryV reduction by an admixture of steam and carbon dioxide or preferably because high concentrations of H2O `and/or CO2 result spontaneously from a low rate of gas flow. The volatilization of Zinc is particularly strongly suppressed when the gas flows at a low rate and if the concentration of CO2 and/ or H2O at the gas outlet of the preliminary reduction furnace is so high as to exceed the equilibrium concentrations of the equilibria atv the preliminary reduction temperature. in that casey no metallic iron will be formed in that preliminary reduction zone. if

The reduction of Zinc oxide is preferably performed' in a vacuum after the preliminary reduction and/or the admixture of low-oxygen iron oxide and/or iron. The

degree of the vacuum and the reduction temperature are.- preterably adapted to each other to shorten the duration.

of the treatment. The higher the vacuum. the lower temperatures will be suhcient to obtain a removal of' zinc inV a comparable time. For instance, a reduction at 900 deg. C. in a vacuum of 0.1. mm. Hg will lead tov the saine degree ofzinc removal in about the same time` as a reduction at a temperature of 1050 deg. C. at 10 mm. Hg.` Evacuation below 0.01v mm. Hg Will not permit of a further reduction of the temperature.

, The reduction of zinc oxide can also be acceleratedI by passing an inert gas, e. g. nitrogen, over the charge.

therewith.' The' removal of zinc in a stream of inertA gas will generally require higher reduction temperatures Vthan in a high vacuum. For this reason ythe operation in agas streamis preferably performed at about 1050 deg.` C. i l

If the reduction of aine oxide is accelerated by passing an inert gas stream thereover, this will involve a considerable loss of heat because large quantities of inert gas must continuously be reheated. Even if the inert gas is recycled the heat of the used inert gas cannot be utilized because the condensation of the zinc will also involve a cooling of theinert gas. In that process the zinc will be obtained in the form of a powder, not in a compact form as when the process is carried out in a vacuum. Thus the condensation of the zinc from the inert gas is rendered more `dillcult and may require electric gas cleaning devices. Therefore, the processing in an inert gas stream has considerable disadvantages compared to the processing in a vacuum. If the material to be dezincked is not in a form of briquettes or in a sintered form, the strong nitrogen stream will promote the formation of dust and, therefore, the contamination of the zinc. Another disadvantage of the processing in an inert gas stream, e. g., in processing calcine from Meggen` pyrites, resides in the fact that sintering oc curs at an early stage during the dezincking ina nitrogen stream, which requires a heating above 1000 deg. C. for several hours. As a result the removal of zinc is incomplete and the product tends to build up at the walls of the dezincking furnace.

When the material to be dezincked contains some carbon or moisture, the nitrogen cycle will tbe enriched with CO2 and H2O and it will be necessary to clean the gas. Moreover, the nitrogen must be free from oxygen and the nitrogen cycle must be protected against an entrance of air` if oxide-free zinc is to be obtained and a re-oxidation of the iron obtained by the preliminary reduction is to be avoided.

The removal of zinc may also be considerably accelerated by agitating the material to be dezincked so that each particle of the material will come to the surface of the material and the zinc vapor can more easily escape from the material.

When the material to be dezincked isheated indirectly from the outside during the removal of zinc the movement of the material will also facilitate the transfer of heat and, therefore, the removal of zinc, which is an cndothermic process.

The process according to the invention is applicable to very different kinds of material. It is particularly suitable for the removal of zinc or for the recovery of zinc and iron from materials in which zinc and iron are contained in the form of oxides or in a form which can easily be transformed intooxides and in which the mole ratio of iron and zinc is such that the molar weight ratio of Fe:ironeombined oxygen can easily be adjusted to 1:1 or 1 in the preliminary reduction stage without appreciable losses of zinc and `a suicient quantity of divalent iron, i. e. iron in the form of free FeO, is obtained for the reduction of the zinc oxide.

Where the starting materials do not contain enough iron for such a preliminary reduction, they may, have iron oxides admixed thereto before the preliminary reduction stage. Preferably a product is chosen in which the ratio of iron` to iron-combined oxygen is between that of Fe30`4 and Fe. In this way, e. g., a material which contains zinc oxide and has a low iron content or contains no iron can be processed according to the invention to obtain zinc and ironl If the iron:oxygen ratio of the added iron oxide is adjusted in accordance with the requirements of the invention and if said iron oxide is added in an appropriate quantity the preliminary reduction stage may be omitted.

On I the other hand, in those cases in which the ratio ironzzinc is not sufficient for forming in the preliminary reduction stage the amount of FeO required for the reduction of zinc oxide it is not necessary to add ironcontaining material in 'order to obtain the necessary amount of iron. It is also possible to etect a preliminary reduction to obtain the optional reduction of` the iron oxide, then to reduce part of the zinc oxide with the iron oxide formed, whereby the zinc is recovered, whereafter the oxidized iron oxide is reduced in a repeated preliminary reduction stage and the reduction of zinc is repeated. It may be advantageous to remove the zinc in such a process in which two stages are performed twice, because in that case smaller quantities are processed in the individual process stages, no mixing is required and higher concentrations of zinc are possible in the first dezincking stage.

When the zinc-containing material to be processed is not in the form of oxides, the form of oxide may be obtained by a pretreatment. For instance, sulfide materials may be roasted. In this case it is necessary to provide for a removal of sulfur to a considerable extent and to re-roast the calcine, if necessary, in order to enable the recovery of pure zinc and of sulfur-free iron. Re-roasting may be effected on the sintering conveyor. This will cause an agglomeration of the material at the same time, which is of special advantage for the recovery of the iron and does not disturb the removal of zinc unless it proceeds too far. On the contrary, that agglomeration may atord the advantage of reducing the formation of dust and the resulting contamination of the zinc. In that connection the temperature rise resulting from roasting may be utilized for the preliminary reduction land the reduction of zinc oxide if the preliminary reduction is performed with a gaseous reducing agent or if the calcine or sintered material contains unburnt fuel in the amount required for the preliminary reduction.

Various apparatus may be used to effect the reduction whereby divalent iron is formed. For instance, the carboncontaining charge can be heated in an externally heated furnace, e. g. a retort-type shaft furnace or rotary kiln. If the material obtained on the sintering apparatus contains unburnt fuel, the process according to the invention can be performed in an embodiment which is particularly simple `and advantageous from the thermal aspect: The hot sintered material is filled into crucibles, which are heated in a covered condition 'for some hours in the reverberatory furnace to about deg. C. and are sub sequently placed in an uncovered condition into an evacuable chamber, in which the zinc is removed.

If the starting material comes from the sintering conveyor or from a calcining furnace or the like, it may be flown through by a reducing gas on a belt device or on a step grate unit. Such units are known for the air-cooling of granular materials. For the purpose according to the invention the air is replaced by a reducing gas. To complete the reduction the reducing gas may be recycled and may be additionally heated.

The reduction with the aid of gaseous reducing agents may also be performed in a rotary kiln. The same may either be heated externally `and serve also for removing the zinc, or a rotary kiln may be used which permits of drying and calcining and/or removing sulfur from thc starting material in addition to the reduction thereof. To this end blast furnace gas may be introduced at the lower discharge end of the inclined revolving pipe, which gas is burnt in the middle of the rotary pipe by air introduced there. Thus the material fed in the opposite direction is heated, dried, calcined and has sulfur removed therefrom. In the lower part of the drum the hot material interacts with the blast furnace gas and is thus reduced. However, it is also partly dezincked. The reduction and the previous drying, calcining removal of sulfur and sintering which may be necessary are preferably performed in shaft furnaces or may be effected in a story furnace. In lshaft furnaces the filtering action of the charge on the zinc vapor and zinc oxide vapor reduces the removal of zinc compared `to the treatment in a rotary kiln. The large shaft furnace required in view of the low velocity of reduction is less expensiveA in prime costaud maintenance than a rotary kiln of the same capacity.

The shaft furnace is also preferablyr operated in suclr a manner that its upper part serves as a dead-roasting and heating zone and its lower part acts as a preliminary reduction zone. To this end reducing gas is fed from below through the charge, which travels Yfrom the top to the bottom and which is continuously 'fed at the top and is continuously withdrawn at the bottom. Hot air or flame gases containing a surplus of air are blown into the shaft furnace between the two zones.

The removal of zinc is performed in the vacuum or in a nitrogen stream, preferably in a high vacuum (l mm. to -3 mm. Hg). kThe velocity of zinc removal increases with the tempera-ture and with the reduction o-f the gas vpressure in the, vessel, decrease of the length of path, increase of the cross-'section of the passage for the flow of the zinc vapor 'to the condenser and decrease of the condenser temperature. When the dezincking vessel is evacuated by a rotary oil pump to (m5-0.1 mm. Hg and the temperature is 600 deg. C., a charging of a material containing FeO and zinc oxide, will soon cause the formation of a zinc plating in the cold part of the vessel. At a temperature of only 800 deg. C. more than 80% of the zinc can be recovered within a few hours in the form of compact, fusible, oxide-free, zinc, which is flexible and can in most cases also berolled. Compact zinc is obtained even at condensing temperatures below the melting point of zinc, e. g., at room temperature. f

Since only a low temperature isrequiredthe derzincking vessel may be constructedras a furnace made of iron or steel and the same may be heated from the outside by means of inexpensive fuel of low calorific value, e. g., with blast furnace gas.

If the material to be dezincked has absorbed enough heat, e. g., during the preceding reduction and its zinc content is low or a slight removal of zinc is suicient, the additional heating during the removal of zinc may be omitted if the heat capacity of the material is sufficient for supplying the heat required for the endothermic removal of zinc. This variant of the process according to the invention may serve particularly for the removal of zinc from iron ores having a low zinc content. It is known that many Airon ores contain a few tenths of a percent of zinc,A which cause difficulties in blast furnace operation.

Additional heat carrying members, e.V g., in the form of heated corundum balls, maybe added to the furnace The work to be performed by the vacuum pumps is only small if the reduction is performed preferably with the aid 'of a small amount of carbon, which is almost completely consumed, or with the aid of gaseous reducing agents in such amanner that carbon monoxide or carbon dioxide cannot form during the removal, of zinc. Since thematerial has preferably been dried, calcined, and freed from residual sulfur before, steam, carbon dioxide or sulfur dioxide will not be formed during the vreduction of zinc oxide in appreciable quantities which would. deteriorate the vacuum and require a permanent operation of the pumps, which would involve an expenditure ofY considerable amounts of energy to maintainl the relatively high vacuum required.

Since relatively short dezincking periods will be sucient with a high vacuum and wide cross-section for the stream of zinc vapor at a relatively low temperature, at which iron and steel are still stable, the dezincking vessel is preferably designed as an unlined rotary kiln, which is heated from the outside. In that case the heat transfer. to the material is good in spite of the high vacuum. The furnace can easily be constructed as a vacuumtight unit. It may be closed at one end and have at the other end a cover with a quick-acting closure, which lies outside the combustion zone. The zinc will condense before they cover.

Fig. 2` shows by way of example an apparatus which vso ' enlarged portion a l0 maybe -used for reducing the zinc oxide. lltV comprises aniron` pipe 1, which is closed at one end :Zand` whose other end 3 is connected to a vacuum pump. The closed heated part of the pipel is surrounded by a casing 4,r which can be evacuated at 5. The casing 4 is rotatably carried at 6 and 7 by means of wheel rims in the manner of a rotary drum. The pipe 1 can be heated by means of electric heating coils S. The pipe 1 contains the prereduced material, which contains zinc oxide and iron oxide in a mixture, at the closed end and in the heated zone. After the vessel 4 has been subjected to a vacuum, the tube 1 is heated upby the electric heater 8 and the vacuum is connected also at 3. The zinc formed by re,- duction in the heated mixture passes into the air-cooled part outside the Vessel 4, where it condenses at 10 as cornpact zinc. The pIatesSuSpended at 11 are shields preventing la radiation of heat from the heated part of the pipe. 12v is insulating quartz glassv wool.

In that device it is possible to form the pipe 1 of steel which is not heat-resisting and with a small wall thickness, because it is protected against the outer air pressure-by the vacuum in the container 4. For this purpose the vacuum in the container 4 need not be particularly high. It is also recommendable to use a high vacuum in this case, however, because. the same provides for al better then remain ineffective.

It is favorable to permit the zinc to condense at the lowest temperature possible because in that'case its vapor pressure will bev low and. solid, compact Yzinc will be directly deposited. in that case Athe condensation may take place, e. gJon inserted iron plates, which may be placed intothe remelting furnace together with the zinc built up thereon.

On the other hand,4 if the condensing temperature is chosen above the melting point, preferably only slightly above the same, the end of the pipe 1 protruding from the container 4 may be axial-symmetrically enlarged. In the sump of molten zinc will be formed, which will promote the removal of any partly oxidized zinc particles during the rotation of the kiln. in that caseV it is. desirable to use a moderate vacuum in order to avoid ran evaporation of the liquid zinc.

The foregoing pressure 'data refer to the pressure measured behind the condenser viewed in the direction of the 'conduit leadingto the pump.

most expensive grinding to finest particle size. The'proc-r ess according to the invention is also particularly suitable for minerals, in which the Fe:Zn mole ratio is at least 3:1. For this reason the process is particularly suit-V able for the calcine from Meggen pyrites, which complies with both saidrequirements.

The process according to the yinvention is not only applicable'tokmaterials which contain zinc oxide as such, but also to materials which contain zinc oxide combined with iron oxide, silica and the like and in which the zinc oxide can be reduced by FeO in a vacuum at an elevated temperature, with formation of Zinc. For this reason these materials are also referred lto as oxide materials.

Some examples will be described hereinafter to facilitate the understanding of the present invention. First the principle of the process may be shown in an experiment.

According to the equation.

the said mixture was repeatedly rapidly heated to 1000 deg. in a vacuum, rapid cooling and comminuting,

until the material contained no more than 0.05% ofv 1 1 metallic `iro'n and` 78.38%` FeO. The determination of theinctallic iron was made bythe HgfClz method, the determination of the divalent iron by titrating according to Reinhard-Zimmermann in the absence of air.

. 20 grams of the material were homogeneously mixed with 4.44 grams zine oxide. 19.70 grains of the resulting mixture were weighed into a small bowl for the removal of zincin a vacuum. The removal of zinc in a vacuum was .effected in a quartz pipe closed at one end. The pressure was 0.1 mm. Hg, the total dezincking time l hours, the temperature 900 deg. C. during hours and 1100 deg. C. during ve hours. After a temperature of 560 deg. C. had been reached during the rapid heating of the quartz pipe the `beginning of the condensation of zinc in the cold part of .the quartz pipe was to be observed.y At 750 deg. C. a thick plating of zincofibright, silverlilce appearance had beenformed inthe air-cooled part of the quartz pipe. After the experiment had been terminated it was possible to remove most of the zinc in the form `of a llexible sheet material from the pipe. A total of 2.87 grams of zinc were weighed out. By calculation, only 12.5 milligrams had been volatilized under the action `of the 0.05% of metallic iron. Thus 99.3% of the zinc had been volatilized by the action of the divalent iron oxide and obtained as metallic zinc.

The process according to the invention may be explained with reference to the following examples:

r Example 1 A mixture of Fe2O3 and ZnO, weight ratio 5:1, was reduced at 800 deg. C. by a gas which consisted of 50% carbon monoxide `and 50% carbon dioxide (parts by volume). grams of the substance were contained in a small bowl, which had been placed in a pipe through which the gas was passed at a rate of 2l litres per hour. Afterthe pipe had been heated to 800 deg. C. the gas was passed therethrough for two hours. The quenched product obtained by this preliminary reduction had the following analysis: 0.00% by weight metallic Fel 73.08%by weight divalent Fe related to the iron con- 26.92 by weight trivaleut Fei tent of the Sample 96.64% by weight oi the zinc content of the charge It has `thus been shown "that the considerable reduction of the iron content to form divalent iron is possible at 800 deg. C. without appreciable volatiliaation of the zinc.` A considerable Volatilization of zincrwas observed at higher temperature, particularly above 950 deg. C., and a higher ow rate.

This preparation, which did not contain metallic iron, was subjected to a vacuum dezincking step as in the basic experiment. In this case, however, the temperature was maintained at 500 deg. C., 700 deg. C. and 800 deg. C. for five-hour periods and the pressure was 0.05 mm. Hg. During that treatment a total of 74.5% of the zinc content were distilled oi and collected, namely,

7.6% by weight during the treatment at 600 deg. C. 35.6% by weight during the treatment at 700 deg C. 56.8% by weight during the treatment at 800 deg C.

In this case it was also possible to remove the zinc from the pipe in the form of a compact sheet ring.

Example 2 Calcine from Meggen pyrites had the following analysis after being sintered on the conveyor:

The sintered calcine was reduced with a reducing gas consisting of 50% by volume CO and 50% by volume 12 COgat 800 deg. C. In view of the' knownposition 'of the reduction equilibria no formation of metallic iron was to be expected under these conditions. The `ow rate of the gas' was 0.8 litre per hour, the duration of the test four hours, the temperature 800 deg. C. and the charge 1.15 grams of dried sintered calcine. No zinc was volatilized during the preliminary reduction. The prereduced product did not contain metallic iron.

This was followed by dezincking in a vacuum:

Vacuum mm. Hg-- 0.1

Temperature deg. C 1000 Time hours-- 4.5

Yield of zinc percent- 100 Example 3 Calcine from Meggen pyrites had the following composition:

9.98% by weight Zn 41.0% by weight Fe 7.5% by weight total sulfur content 1.3% by weight sulfur in the form of sulfate The calcine was introduced into a vertical Pythagoras pipe 60 mm. wide, which was heated to 900 deg. C. in an electric-coil furnace. The charge rested on a screen plate. The height of bed was 50 em. Air was blown in through the screen plate until no S03 fumes escaped any, longerat the upper end. The sulfur content of the dead-roasted calcine was only 1.20% total sulfur content andV 1.03% sulfur in the form of sulfate. 37 grams of the deadroasted calcine were subjected in a bowl to the preliminary reduction with town gas, to which steam had been admixed, at 800 deg. C. The ilow rate of the gas was 15 liters per hour, the duration of the experiment was 3 hours. 3.7% of the zine were volatilized. The pre-reduced product contained 47.6% metallic iron and 52.4% divalent iron, related to its total iron content. Then the zinc oxide was reduced by heating for live hours at 900 deg.vC. vunder a vacuum of 0.1 mm. Hg. 94% of the zinc contents were deposited in the cold part of the dezincking furnace in the form of a compact sheet zinc ring. During the preliminary reduction the sulfur in the form of sulfate is transformed into sulfide. No sulfur is volatilized during the dezincking stage and the zinc isfree of sulfur.

Example 4 The calcine which isfdevad-roasted in a shaft lfurnace as described in Example 3 was subjected to preliminary re duction for nine hours at SOOVdeg. C., the air stream being replaced by a stream of town gas at liters per hour. The' towngaswas under a pressure of 60 cm. water column.' Only 0.3% of? the zinc content were volatilized. The prereduced` product contained 73.10% divalent iron andlll86% metallic iron; related to the total iron content, balance trivale'ntiron.V 100 grams of the pre-rcduced product were `introduced into a rotary pipe which is electrically heatable from the outside and were heated for five hours to 900 deg. C. The vacuum amounted to 0.1mm; to 0.01 mm. Hg. The pipe was alternatingly given three left-hand and three rightlhand turns to enable the evacuation of the pipe by means of a'rubber hose. The removal of zinc amounted to 83.7%. T he zinc was obtained as a compact sheet ring. in aneapcrimcnt identicalA inv other respects but performed without rotation'of thekiln the removal ofzinc amounted only to 31.8%;

Example 5 A Sintcred calcine from Meggen pyrites, having the composition described in Example 2, was subjected-to pre liminaryA reduction for 2:2 hours at a temperature of 800 deg. C. by passing hydrogen thereover, to which some steam hadrbeenadmixed. Only 0.7% of the zine'content were volatilized. The pre-reduced product contained 74.29%. metallic iron and 20.26% divalent iron, balance trivalent iron, related to the total iron content. The zinc oxide, reduction` was performed by heating for six hours at 800 deg. C. in a vacuum of 0.1 mm. Hg andV yielded 74% of the zinc inthe form of compact zinc.

` i .Ermmvlffy 6 rA preliminary reduction was performed in. a` shaft furnae; The shaft furnace was charged from the top with Calcine from Meggen pyrites. That calcine had previously been dead-roasted on tlat pans at 900 deg. C. in the presence of air-and' with frequent re-shovelling. It had a particle sizeof 10 mm. and the lfollowing analysis:

42.16% by weight total iron content 9.72% by weight zinc 1.52% by weight total sulfur content 1.1% byweight sulfurv in the form of sulfate Conditions of the preliminaryfreduction:

Temperature, 800 deg.` C. Time of stayin the shaft furnace, 18 hours Rate of lflow `of town gas fed from below in a countercurrent against the charge, 80 liters per hour Width of shaft'opening, 60mm.

Heightof bed; approximately 0` cm.

` Weight loss during the preliminary reduction, 28%

Analysis after the preliminary reduction:

52.50% by weight total iron content, consisting of 51.9% 'byvl weightmetallic iron 47.3 by'weightgdivalent iron f 0.81% jby weight trivalentfiron 11.36% by weight zinc Example 7 vPrerednced material from the same charge as used in Example 4 was dezincked in a nitlOgen stream. 65.58v

grams ofthe material were contained in a bowl, which had been placed in the middle part of a quartz glass pipe 80 cm. long kand 40 mm. wide. .That part had been heated' to 9.00`deg. C. The nitrogen containedless than 0.01%l byvolume O2 and had a ow rate of 20 liters per hour. The duration of' experiment was live hours. The weight loss of the material during the dezincking treatment was 1.362 grams, torresponding to 16.5% vVolatilized zinc. v

A comparison with Example 4 shows that the dezincking in a vacuum at the same temperature provides for a higher degree of zinc removal during-the same time.

Example 8 Sintered calcine f rom Meggen pyrites, having the analysis `stated-in yExample 2, was heated for two 'hours to 1000fdegC. inthe absence of air, whereby the content of carbon was reduced to.-0.l% and the content of divalent Aironwas increased tor 55.35% (related tothe total iron content). No content of metallic `iron was ascertained. During that preliminary reduction 1.22%v of the Zins:` content passed koff from the sinter.v Thereafter the material walsskeptfor rfvehours, at 900 deg. C. under a pressureof 0.2v mm. Hg, whereby 44.7% of the zinc content were sublimed off. A maximum 0f'. `53%y was to be; expecltedyaccolrding toFig. 1. Y .Y j

14 Example 9 v17.52 grams zincorthosilicate Zn2SiO4 were ground together with 60.7 grams of a ferrous oxide preparation and were heated for five hours at 900 deg. C. in a vacuum of 0.2 mm. to 0.05 mm. Hg. During that treatment 48.9% of the zinc content of the silicate condensed inl the form of compact, metallic zinc of high purity on the inside wall of the vacuum retort outside of the furnace.v The residue was ground and reheated under the sameconditions.l A total of 93.3% of the zinc wasobtained in metallic form. Y

The zinc silicate usedwas prepared from a mixture of analytically purev zinc oxide powder with a 5% surplus of highly dispersed silicon dioxide by heating for tive hours at 1200 deg.y C., grinding, andfreheating. Thev Debye-Scherrer picture showed no ZnO lines and corresponded to pure willemite.

The ferrous oxide used wasprepared by repeated grinding and heating of pure iron powder and powdered ferrie oxide to temperatures up to 1300 deg. C. lt contained nally no more than 0.09% metallic `iron and 79.7% FeO.

During the production of the FeO preparationl and the reaction with the silicate, rapid quenching and rapid heating were employed to prevent a rearrangement of the FeO to form Fe and Fe304. A partial rearrangement will take place with cooling and heating times as are economical in commercial operation. But even in that case the FeO remains the main reducing agent. The reaction corresponds t-o the equation A Zn2SiO4-|8FeO- 2Zn|FegSiO-Hle0.

Natural willemite can be processed correspondingly.

l Example J0 Zinc-ferrite ZnO.Fe2O3 was prepared from 81.38 grams pure zinc oxide and 159.7 grams pure @e203 by heating three times for several hours at 1200 deg. C. and. grinding. 1

36.87 grams of the ferrite were mixed with 60.7 grams of ythe FeO (as in Example 9)y and were heated twice for vtive hours to 900 deg. C. at 0.2-0.005l mm. Hg, as in Example 9. The removal 4of zincY was complete. It follows the equation f The zinc deposited in a pure, compact form in the unheated partofthe vacuum retort. Natural franklinite can be processedl correspondingly.

Example 1 Calcine from Meggen pyrites, having the compositionstated in Example` 3 was ground in a ball mill. The sieve analysis was Y 18% particle size more than 0.1 mm. 82% particle size less than.0.1 mm. 55% particle size less than 0.06 mm.

After about 6% water had been added the material was easily formed into balls (pellets) on a pelletizing plate. It is remarkablerthat an addition of binder was not` necessary` because the material has an excellent inherent plasticity and the pellets, about 8 mm. in diameter, were as hard as stone after drying at temperatures up to deg. C. f 1

1500 grams of these dried pellets were then deadroaste'd for 45 hours at 800 deg. C. in a shaft furnace 60 mm. wide, through which air was passed at a rate of,A 200 liters per hour without an appreciable pressure drop. The heat required was substantially generated by the combustionr of the residual sulfur content, which caused 'a Weightloss of 81 grams. The dead-burnt pellets showed also a rsurprisingly high strength.

The partial'reduction rof the iron content (preliminary reduction) `was then also 4effected in the shaft furnace. With a charge of 1337 grams it' was performed during 15 9 hours at 900 deg. C., using town gas at 80 liters per hour and gave a product containing 17.8% FeII and 82.2% Fem, related to its total ironl content. vThe volatilization of zinc amounted to 6.6% of the total zinc content.

A similar preliminary reduction performed only for 4.5 hours gave a product in which the ratio of FeII and Femet was substantially reversed. The volatilization of zinc amounted only to about 0.5% and the pellets were much harder so that they withstood not only the subsequent dezincking treatment in the rotary vacuum kiln without disintegration and formation of `dust but showed also during the following smelting in the blast furnace a strength similar to that of sintered ore. Compared to sintered material the pellets when used in the blast furnace ihave the advantage that they. can be reduced more easily and more uniformly and do not contain dust.

Then the pre-reduced pellets were charged into the When the dead-roasting is performed in the upper part and the preliminary reduction is performed in the lower part of a shaft furnace, by feeding a gas capable of reducing to the bottom ofthe shaft furnace and feeding air betweenfthe reducing and dead-roasting zones, the

gas `which `has almost completely been consumed during the preliminary reduction when fed at a low rate can be burnt with preheated air completely and serve to supply the heat required by a material having a low sulfur content, e. g. calcine from a fluidization furnace. Since the combustion of the weak gas is so complete, probably owing tothe presence of the active pellet surfaces, the dead-roastingY is not disturbed.

What is claimed is:

1.l A process for removing zinc from zinc oxide containing `material which comprises reducing the zinc oxide with iron oxide having an Fe:O mol ratio exceeding 3:4 at a temperature above 600. C. and simultaneously removing the zinc formed from said material in vapor form.

2. The process of claim l in which said zinc oxide containing material also contains iron.

3. A process as set forth in claim l, in which the zinc oxide is reduced in a vacuum.

4. A process as set forth in claim l, in which iron oxide having an Fe:O mole ratio exceeding 3:4 is added to said zinc oxide containing material before said zinc oxide is reduced. 4

5. The process of claim4 in which at least the major portion of the iron oxide is in the form of free ferrous oxide. f

6. A process as set forth in claim 1, in which the zinc oxide is reduced in a high vacuum at 850-950 deg. C.

7. A process as set forth in claim 6, in which the zinc oxide is reduced in a vacuum of approximately 0.1 mm. Hg.

8. A process as set forth in claim 1, in which thezinc oxide containing material contains also iron oxide having an Fe: mole` ratio below 3.4 and which comprises reducing said iron` oxide to obtain an Fe:O mole ratio exceeding 3:4 before said zinc oxide is reduced.V

9. A process as set forth in claim 8, in which the reduction .of .the iron oxide is controlled to prevent an acgasdotti companying reduction of susbtantial amounts of zinc oxide. t

10. A process as set forth in claim 8, in which said iron oxide is reduced until substantially the entire iron content of the material is available as Fe:0.

ll. A process as set forth in claim 8, in which said iron oxide is reduced until the iron content of the material is available partly as metallic iron.

l2. A process as set forth in claim 8, in which the iron oxide is reduced with a reducing gas containing at least one substance selected from the group consisting of carbon monoxide, hydrogen, and hydrocarbons.

13. A process as set forth in claim 8,1which comprises reducing the iron oxide by feeding the material from top to bottom through a furnace, passing. reducing gas through said material from the bottom to the top of said furnace, and maintaining said furnace at'a temperature of 800-900 deg. C.

14. A process as set forth in claim 13, in which a shaft furnace is used for reducing the iron oxide.

15. A process as set forth in claim 13, in which the reducing gas contains CO2 in such a concentration that the equilibrium concentration of CO2 in the equilibrium is exceeded in at least part of the furnace.

16. A process as set forth in claim 13, in which the reducing gas contains H2O in such a concentration that the equilibrium concentration of H2O in the equilibrium is exceeded in at least part of the furnace.

17. A process as sety forth in claim 8, in which the iron oxide is reduced by heating the material inthe presence of solid carbon.

18. A process as set forth in claim 17, in which the iron oxide is reduced by heating the material to temperatures above 900 C. at which no substantial vaporization of zinc occurs.

19. Aprocess as set forth in claim 17, in which the iron oxide is reduced by heating the material until the carbon content thereof has been reduced to less than 0.1% related to the weight of the material.

20. A proces as set forth in claim 17, which comprises sintering the material by burning carbonaceous fuel so as to leave a residue of carbon in the sintered material before the iron oxide is reduced.

21. A process as set forth in claim 17, in which the iron oxide is reduced by heating the material in covered crucibles.

22. A process as set forth in claim 17, in which the iron oxide is reduced by heating the material in covered crucibles and in which the crucibles while still hot after the reduction of the iron oxide are exposed in an open condition to a vacuum to cause a reduction of the zinc oxide.

23. A process as set forth in claim 8, in which the iron oxide is reduced until Fell Fenici; 1 riff "rs-*sm M moles Fe 24. A process as set forth in claim 1, in which the zinc oxide is reduced by iron oxide which contains divalent iron in an amount which is more than three times the amount of any metallic iron contained in the material subjected to the treatment employed to cause a reduction of zinc oxide.

25. A process as set forth in claim 1, in which the zinc oxide is reduced in a rotary pipe of iron, which is closed at one end and is heated at this end and for part of its length whereas it is cooled and connected to a vacuum at the other end.

26. A process as set forth in claim 25, in which the pipe is heated from the outside and a vacuum is provided around the heated part of the pipe.

27. A process for removing zinc from material which contains zinc in the form of compounds selected from the group consisting of sulfides, sulfates, and carbonatos, which comprises treating the material to convert at least part of the zinc content thereof toznc oxide and reducing said zinc oxide with iron oxide having an Fe:O mole ratio exceeding 3:4 at a temperature above 600 C. and simultaneously removing the zinc formed from said maten'al in vapor form.

28. A process as set forth in claim 27, in which the material is treated to convert the zinc thereof into zinc oxide at least to such an extent as to avoid a formation of gas other than zinc vapor from said material during the treatment employed to reduce the zinc oxide.

29. A process as setforth in claim 1, which comprises pelletizing the material before it is treated to reduce the zinc oxide.

30. A process as set forth in claim 29, in which iron oxide is added to the material before it is pelletized.

31. A process as set forth in claim 29, in which a reducing agent is added to the material before it is pelletized.

32. A process as set forth in claim 29, in which burnt lime is added to the material before it is pelletized.

33. A process as set forth in claim 1, in which a material is used which consists of pellets which are suitable as a blast furnace charge after the removal of the zinc.

' References Cited in the le of this patent UNTED STATES PATENTS 1,751,778 Von Girsewald et al. Mar. 25, 1930y 2,011,400 Eulenstein et al. Aug. 13, 1935 2,144,914 Debuch Jan. 24, 1939 

1. A PROCESS FOR REMOVING ZINC FROM ZINC OXIDE CONTAINING MATERIAL WHICH COMPRISES REDUCING THE ZINC OXIDE WITH IRON OXIDE HAVING AN FE:O MOL RATIO EXCEEDING 3:4 AT A TEMPERATURE ABOVE 600*C. AND SIMULTANEOUSLY REMOVING THE ZINC FORMED FROM SAID MATERIAL IN VAPOR FORM. 